If you didn't take a crack at Mega Man Christmas Carol when it released two years ago, give it a shot now (.zip, 16 MB). The fan-made game reimagines the Ghosts of Christmas (plus miserable old Marley) as boss showdowns in Mega Man. Sprites, Inc. has remade its first game with a new engine, and continues to tweak the difficulty based on community feedback.

The video above should give you a good idea of what it's all about. Future seems to be the toughest Ghost/Master, from the discussion in that forum thread. (The game's five difficulty levels go up to a "Wut" mode, good luck with that.)